1. Field of the Invention
This invention relates to an energy subtraction processing method and apparatus. This invention particularly relates to an improvement in alteration of parameters for energy subtraction processing.
2. Description of the Related Art
Techniques for photoelectrically reading out a radiation image, which has been recorded on a photographic film, in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, and the X-ray image is photoelectrically read out from the X-ray film, an electric signal (i.e., an image signal) being thereby obtained. The image signal is converted into a digital image signal. The digital image signal is then processed and used for reproducing the X-ray image as a visible image on a photocopy, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like, can be reproduced.
Further, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
In order for an image signal to be detected accurately, certain factors which affect the image signal must be set in accordance with the dose of radiation delivered to the stimulable phosphor sheet, and the like. Novel radiation image recording and reproducing systems which accurately detect an image signal have been proposed. The proposed radiation image recording and reproducing systems are constituted such that a preliminary read-out operation (hereinafter simply referred to as the “preliminary readout”) is performed in order to approximately ascertain the radiation image stored on the stimulable phosphor sheet. In the preliminary readout, the stimulable phosphor sheet is scanned with a light beam having a comparatively low energy level, and a preliminary read-out image signal obtained during the preliminary readout is analyzed. Thereafter, a final read-out operation (hereinafter simply referred to as the “final readout”) is performed to obtain the image signal, which is to be used during the reproduction of a visible image. In the final readout, the stimulable phosphor sheet is scanned with a light beam having an energy level higher than the energy level of the light beam used in the preliminary readout, and the radiation image is read out with the factors affecting the image signal, which have been adjusted to appropriate values on the basis of the results of an analysis of the preliminary read-out image signal.
The term “read-out conditions” as used hereinafter means a group of various factors, which are adjustable and which affect the relationship between the amount of light emitted by the stimulable phosphor sheet during image readout and the output of a read-out means. For example, the term “read-out conditions” may refer to a read-out gain and a scale factor which define the relationship between the input to the read-out means and the output therefrom, or to the power of the stimulating rays used when the radiation image is read out.
The applicant proposed various methods for setting the read-out conditions without the preliminary readout being carried out. The methods are described in, for example, U.S. Pat. Nos. 4,284,889 and 4,346,406. With the proposed methods for setting the read-out conditions, a stimulable phosphor sheet, on which a radiation image has been stored, is exposed to radiation, and light, which is emitted instantaneously from the stimulable phosphor sheet, is detected with a photodetector. Information, which represents the characteristics of the radiation image, the amount of energy stored on the stimulable phosphor sheet during its exposure to the radiation, or the like, is obtained from the instantaneously emitted light. The read-out conditions are then adjusted in accordance with the obtained information.
Regardless of whether the preliminary readout is or is not carried out, it has also been proposed to analyze the image signal (or the preliminary read-out image signal) obtained and to adjust the image processing conditions, which are to be used when the image signal is processed, on the basis of the results of an analysis of the image signal. The term “image processing conditions” as used herein means a group of various factors, which are adjustable and set when an image signal is subjected to processing that affects the gradation, sensitivity, or the like, of a visible image reproduced from the image signal. In the systems wherein the preliminary readout is not carried out, the image processing conditions also include the read-out gain and the scale factor, which serve as the aforesaid read-out conditions. The proposed method is applicable to cases where an image signal is obtained from a radiation image recorded on a recording medium such as conventional X-ray film, as well as to systems using stimulable phosphor sheets. (In this specification, the system for adjusting the read-out conditions and/or the image processing conditions will often be referred to as the EDR processing system or the EDR processing means. EDR is an acronym for an exposure data recognizer.)
In the radiation image recording and reproducing systems wherein recording media, such as X-ray film or stimulable phosphor sheets, are used, subtraction processing techniques for radiation images are often carried out on image signals detected from a plurality of radiation images of an object, which have been recorded on the recording media.
With the subtraction processing techniques for radiation images, an image is obtained which corresponds to a difference between a plurality of radiation images of an object recorded under different conditions. Specifically, a plurality of the radiation images recorded under different conditions are read out at predetermined sampling intervals, and a plurality of image signals thus detected are converted into digital image signals which represent the radiation images. The image signal components of the digital image signals, which components represent the image information recorded at corresponding sampling points (i.e., picture elements) in the radiation images, are then subtracted from each other. A difference signal is thereby obtained which represents the image of a specific structure or part of the object (hereinbelow also referred to as the pattern of a tissue, a structure, or the like) represented by the radiation images.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing technique or the so-called energy subtraction processing technique. With the temporal (time difference) subtraction processing technique, in order for the image of a specific structure (for example, a blood vessel) of an object to be extracted from the image of the whole object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure (for example, a blood vessel) of the object is enhanced by the injection of contrast media. With the energy subtraction processing technique, such characteristics are utilized that a specific structure of an object exhibits different levels of radiation absorptivity with respect to radiation with different energy distributions. Specifically, an object is exposed to several kinds of radiation with different energy distributions. Alternatively, the energy distribution of the radiation carrying image information of an object, is changed after it has been irradiated onto one of a plurality of radiation image recording media, after which the radiation impinges upon the second radiation image recording medium. In this manner, a plurality of radiation images are obtained in which different images of a specific structure are embedded. Thereafter, the image signals representing the plurality of the radiation images are weighted appropriately and subjected to a subtraction process in order to extract the image of the specific structure. The subtraction process is carried out with Formula (3) shown below. The applicant proposed novel energy subtraction processing methods using stimulable phosphor sheets in, for example, U.S. Pat. Nos. 4,855,598 and 4,896,037.Sproc=Ka·H−Kb·L+Kc  (3)wherein Sproc represents the energy subtraction image signal obtained from the energy subtraction processing, Ka and Kb represent the weight factors, Kc represents the bias component, H represents the high energy image signal representing the radiation image recorded with the radiation having a high energy level, and L represents the low energy image signal representing the radiation image recorded with the radiation having a low energy level. (The group of Ka, Kb, and Kc will hereinbelow be referred to as the parameters for the energy subtraction processing.)
In the aforesaid energy subtraction processing, when an object is exposed to radiation having a predetermined energy distribution in the course of recording radiation images of the object, the levels of radiation transmittance vary for different thicknesses of the object. Also, the object exhibits a lower level of radiation transmittance with respect to the low energy components of the radiation than the high energy components thereof. Therefore, as the radiation passes through the object, the energy distribution of the radiation shifts to the high energy side as a whole. Such a phenomenon is referred to as the “beam hardening.”
For example, in cases where quantitative determination of a bone mineral in a bone is carried out with the energy subtraction processing, even if the thickness of the bone in the object is the same, the problems described below will occur. Specifically, if the thickness of the soft tissue surrounding the bone is large, the effects of the beam hardening phenomenon will be large, and therefore the image density of the bone pattern in the object image will become low. If the thickness of the soft tissue surrounding the bone is small, the effects of the beam hardening phenomenon will be small, and therefore the image density of the bone pattern in the object image will become high.
If the image density of the extracted tissue pattern fluctuates due to a difference in level of effects of the beam hardening phenomenon, adverse effects will occur on the accuracy of the diagnosis, or the like. Particularly, in cases where an energy subtraction image, which was obtained from energy subtraction processing in the past, and an energy subtraction image, which has currently been obtained from energy subtraction processing, are compared with each other, if there is a difference in level of effects of the beam hardening phenomenon between the two images, there will be the risk that a diseased part is judged by mistake as having been cured.
Further, an energy subtraction image is liable to be affected by scattering of the radiation in the object, and the image quality of the energy subtraction image changes in accordance with the degree of the radiation scattering. Therefore, in order for the image quality of the energy subtraction image to be kept good, it is necessary for the effects of the radiation scattering in the object to be considered.
Therefore, the applicant proposed an energy subtraction processing method wherein, such that the adverse effects of the beam hardening phenomenon may be suppressed, each of the aforesaid parameters Ka, Kb, and Kc for the energy subtraction processing is altered independently in accordance with the thickness of an object or predetermined image processing conditions having a correlation with the thickness of the object. The proposed energy subtraction processing method is described in U.S. Pat. No. 6,075,877.
However, of the parameters described above, the parameter Kc acting as the bias component is the parameter defining the image density shift quantity over the entire area of the energy subtraction image. Therefore, if the parameter Kc is altered independently of the other parameters, there will be the risk that the image density of the entire area of the energy subtraction image will change in accordance with the level of the beam hardening phenomenon.